Efficiency of linear alternator

Interesed in practical maximum efficiency of linear alternators. Suppose an object with a permanent magnetic attached moves along a linear track with a constant velocity, such that its kinetic energy is 1 joule. The object "collides" with a linear alternator or generator, such that the magnet interacts with the coils in the alternator. How much of the initial kinetic energy of the object could be transformed to electrical energy? Am I right in assuming about .25 joules maximum?

You talk about the efficiency of such a system. In my opinion, there is no difference to a conventional rotating alternator, and such systems do have an efficiency that is better than 95%. So, you should get at least some .95 joules and only .05 joules of heat loss. Cheaper systems may be around 80% efficiency. But for an efficient free piston engine, you need more. I calculated some 90% for my prototype.